1. Field of the Invention
The present invention relates to a feeder circuit. The feeder circuit to which the present invention pertains may be employed, for instance, as a subscriber circuit in an electronic exchange or as an office channel circuit in a digital subscriber line transmission system.
2. Description of Related Art
In a normal telephone communications network the subscriber""s terminal unit and the exchange""s subscriber circuit are connected by a subscriber line. Each subscriber circuit has one terminal unit allocated to it, and each subscriber line has two wires.
The subscriber circuit transmits call signals to the terminal unit, receives and transmits voice signals, feeds direct-current and monitors the fed current, by way of the subscriber line. The feeding and monitoring of the current are performed by a feeder circuit provided within the subscriber circuit.
In a normal feeder circuit, the two wires of the subscriber circuit and the power source are connected by means for a transformer. A transformer is employed in order to xe2x80x98floatxe2x80x99 the wires free of the power source. In this manner the wires are not easily affected by fluctuations in earth potential, and by line damage. This transformer feeds an electric current to one of the two wires and takes it in from the other.
The current which is output from the feeder circuit to the one wire and that which is input to the feeder circuit from the other wire are set at the same value. This is because any imbalance in the values of these currents would render it impossible to use the wires for the transmission and reception of control signals. However, current leaks from both the wires, as a result of which there is a need for means for absorbing the imbalance between the two values of current.
It sometimes happens that a conventional feeder circuit ceases to perform correctly when a high-voltage common-mode signal is impressed in the form of noise. In other words, when a common-mode signal is impressed, it becomes impossible to absorb the imbalance between the currents passing through the two wires, moreover it is no longer possible to employ the wires for the transmission and reception of control signals. For instance, the location of a commercial power line in the vicinity of the subscriber line may easily cause a high-voltage common-mode signal to be impressed. The commercial power line may impress a common-mode signal of 10 volt or more on the subscriber line.
It is possible to control the occurrence of imbalances resulting from common-mode signals without xe2x80x98floatingxe2x80x99 the two wires free of the power supply. However, as has already been pointed out, this means that the wires are easily affected by fluctuations in earth potential, and by line damage.
It is an object of the present invention to provide a feeder circuit wherein the subscriber line is xe2x80x98floatedxe2x80x99 free of the power supply, and imbalances resulting from high-voltage common-mode signals do not easily occur.
To this end, the feeder circuit to which the present invention pertains comprises means for regulating which serves to feed an electric current to a positive output terminal and a negative output terminal; first means for feeding electric current which serves to feed an electric current to the means for regulating; means for regulator control which serves to control the electric current output from the means for regulating; and means for output control which serves to control the median value of the potential at the positive output terminal and the potential at the negative output terminal in such a manner as to ensure that it tallies with the median value of the potential at the positive terminal of the means for feeding electric current and the potential at the negative terminal thereof, while also absorbing any imbalance between the current at the positive output terminal and the current at the negative output terminal.